Memory is an indispensable part of electronic devices today. Multiple variants of memory exist and have led to a myriad of niches for multiple memory concoctions. These memory concoctions reside in an ever-changing technological domain allowing for the categorizing of different memory types: volatile versus nonvolatile; fast versus slow; low capacity versus high capacity; and cheap versus expensive. Memory examples include random access memory (RAM), flash, hard drives, and optical disks. Flash memory dominates the nonvolatile memory market today for handheld and battery-operated devices.
Since the identification of a memristor as a resistance based nonvolatile storage element for nanoscale crossbar arrays, multiple applications for the device have been proposed ranging from memory and reconfigurable logic to neuromorphic learning and secure communication. Memristor, in this context, comprises ionic transport devices where electronic charge of ions or other sort of fundamental mechanisms within an insulating material are modulated to change resistance of the device. This current definition should not limit the scope of this disclosure since other forms of resistive memories where successive memory operations can cause incremental resistance changes as described in this disclosure fall within the scope of resistance-based memory cells, what are popularly termed as memristors. From all the applications, the most promising with respect to product development is the digital memory utilizing memristors as storage elements. A new paradigm with respect to memory is necessary for the continued growth in density of nonvolatile memory for anticipated growth in petascale and exascale computing. The memristor's simple structure, small size compared to transistors, and nonvolatility make it a viable candidate for next-generation memory technology. Memristor memory is a subset of resistive memory since logic states are encoded in the memristor's resistance. Even though resistive memory is a more general term, some problems associated with resistive memory in a crossbar array are also characteristic to the memristor memory. The difference between resistive memory and memristor memory lies in the fact that memristors have a pinched hysteresis loop at the origin, while the more general term, resistive, includes devices such as the one in H. S. Majumdar et al's “Memory device applications of a conjugated polymer: Role of space charges”, J. Appl. Phys., vol. 91, no. 4, (2009) which do not possess this trait. Resistive memory in essence comprises a lump of devices with differing resistance-change mechanisms. The method introduced in this disclosure, hence, may not be applicable to all resistive memory devices, but it is definitely advantageous to memristor memory systems.
The memristor memory presents a solution to difficulties encountered beyond CMOS scaling, but it also introduces various complications to realizing this memory system. The patent database provides a myriad of methods to deal with difficulties (resistance drift, nonuniform resistance profile across the crossbar array, leaky crossbar devices, etc.) that arise from working with these resistive memory elements. These difficulties (problems) are addressed within the database by using correcting pulses to mitigate effect of resistance drift due to normal usage; using a temperature-compensating circuit to counter resistance drift due to temperature variation; using an adaptive method to read and write to an array with nonuniform resistance profile; and introducing diodes or metal-insulator-metal (MIM) diodes to reduce leaky paths within the crossbar memory array. With every proposed solution to counter a problem, there are drawbacks that need to be considered. This disclosure exposes a view that will lead to the realization of memristor-based memory in the face of low device yield and the aforementioned problems that plague memristor memory.
This section provides background information related to the present disclosure which is not necessarily prior art.